Add memory pressure listener to Blob storage

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 866a70e3c673610dab0e7f5b177dea7a35020a68..8cadc162b5599fd5cd3535ed7f15e92cdaa0722f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -70617,6 +70617,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Storage.Blob.SizeEvictedToDiskInKB" units="KB">

[Ilya Sherman, 2017/05/11 20:13:40]

nit: Please add the attribute base="true" to indicate that this histogram name
is not emitted to directly.

[ssid, 2017/05/12 01:38:01]

Done.

+  <owner>ssid@chromium.org</owner>
+  <summary>Records the total size of in-memory blobs evicted to disk.</summary>
+</histogram>
+
 <histogram name="Storage.Blob.StorageSizeAfterAppend" units="KB">
   <owner>dmurph@chromium.org</owner>
   <summary>
@@ -127301,6 +127306,18 @@ from previous Chrome versions.
   <affected-histogram name="Storage.BlobItemSize.BlobSlice"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="Storage.BlobEvictReason" separator=".">
+  <suffix name="OnMemoryPressure"
+      label="Eviction was triggered because of memory pressure signal."/>
+  <suffix name="SizeExceededMaxDiskSpace"
+      label="Eviction was triggered because the total size exceeded maximum
+             available disk space"/>
+  <suffix name="SizeExceededInMemoryLimit"
+      label="Eviction was triggered because the total size exceeded maximum
+             in memory size allwed"/>
+  <affected-histogram name="Storage.Blob.SizeEvictedToDiskInKB"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="StunPingInternal" separator=".">
   <suffix name="0ms" label="0ms between requests"/>
   <suffix name="5ms" label="5ms between requests"/>